1. Field of the Invention
This invention relates to a spread-spectrum communication apparatus.
2. Description of the Prior Art
In a spread-spectrum communication system known in the art, a baseband information signal to be transmitted is converted into a baseband signal having a large bandwidth by a pseudonoise code which possesses a spectrum width sufficiently larger than that of the information signal, the baseband signal is modulated by a modulating method such as PSK (phase-shift keying) or FSK (frequency-shift keying) to form a high-frequency signal, and the high-frequency signal is transmitted. In order that mixing of signals will not occur in this system, it is required that a third communicating party using the same pseudonoise code not exist.
In a case where such a spread-spectrum communication technique is applied to a communication system which accommodates a plurality of terminals, a different spread-spectrum code is allocated for each terminal in order to prevent interference between terminals connected to an extension.
However, since a different code generator is provided for each apparatus, the following drawbacks arise:
1) Since code generators cannot be shared, the code generators result in high cost. This raises the cost of the apparatus.
2) It is very difficult to manage code generators of a large number of types.
Furthermore, in order to maintain synchronism between the transmitting and receiving sides, a synchronizing circuit which includes a correlation detector for each code pattern is required. Consequently, there are different components for each terminal. This leads to an increase in terminal cost and to difficulties in production.
In addition, when a sliding correlator is used in order to readily detect correlation with a received signal, an extended period of time is necessary to achieve synchronization.
Further, when a convolver or the like is used in order to achieve synchronization, the convolver differs for each code pattern. The result is a rise in cost and difficulties in production.